Piezoelectric devices such as crystal resonators have been utilized in communication devices and in other various fields. For example, a conventional surface-mount crystal resonator, as shown in FIG. 21, is composed mainly of a crystal resonator element 10 in which electrodes including an excitation electrode 11a (and another excitation electrode 11b on the backside) are provided on a crystal resonator plate, a holder 3 having a recess 8 for accommodating the crystal resonator element 10, and a lid (not shown) for hermetically sealing the recess 8 by being bonded to the holder 3. The crystal resonator element 10 has a plate-like shape, an end of which is cantilevered and bonded by conductive adhesives 6, 6 on electrode pads 5, 5 inside the recess 8. The crystal resonator having this configuration is disclosed, for example, in Patent Literature 1 and Patent Literature 2.
For example, in the case of an AT-cut crystal resonator plate whose oscillation frequency is inversely proportional to its thickness, the frequency is adjusted by addition of excitation electrodes to the front and back main surfaces of the crystal resonator plate and partial removal of the added excitation electrodes. To start with, excitation electrodes having a predetermined thickness are added to the front and back main surfaces of the AT-cut crystal resonator plate, which decreases the frequency to a range slightly lower than a target frequency. This adjustment is called coarse frequency adjustment. After the “coarse frequency adjustment”, the crystal resonator element is conductively bonded on the inside of the holder. In this state, the excitation electrodes are irradiated entirely, for example, by an ion beam. Ion beam irradiation to the excitation electrodes decreases the mass of the excitation electrodes and increases the frequency to the target frequency. This adjustment is called fine frequency adjustment.